For example, in a motorcycle for a motocross race, because of its use, the motorcycle may hits a road surface hard due to landing from a high place at the time of jumping. Thus, it is required to increase rigidity and strength of a body frame against impact, and it is also required to reduce weight of a body as much as possible.
As a body frame of this type, as shown in FIG. 13, conventionally, a structure may be adopted in which a main pipe (tank rail) 80 of iron extending obliquely downward to a rear of a vehicle is welded and joined to an upper end of a head pipe (not shown) of iron and a down tube 81 of iron also extending downward to the rear of the vehicle is welded and joined to a lower end thereof, and left and right gussets 82, 82 are welded and joined to front side parts of the down tube 81 and the main pipe 80, whereby the head pipe part is closed in a section thereof. In addition, a guide pile 83 opening in a direction of the vehicle width may be welded and joined between the left and right gussets 82 to arrange a clutch cable 84 or the like in the guide pipe 83
Incidentally, in the structure in which iron pipes are welded and joined as in the conventional body frame, although strength can be secured, there is a limit in realizing reduction of a body weight.
On the other hand, recently, from the viewpoint of realizing reduction in a body weight while securing strength, it has been discussed to manufacture a body frame from aluminum alloy, and this has been partly to put to practical use. In the case in which this body frame made of aluminum alloy is adopted, in order to secure strength equal to or larger than that of the conventional iron pipe, it is conceivable to weld and join an aluminum pipe material formed by extrusion molding.
In addition, as a general body frame for a motorcycle, there is one which has a structure provided with a head pipe unit constituted by joining a frame member to a head pipe, a down tube extending downward to a rear of a vehicle from a front part at lower end of the head pipe unit, and rear arm brackets extending in a vertical direction, which joins a rear end of the down tube and a lower end of the head pipe unit, and an engine unit is mounted in this body frame.
Further, as the down tube, conventionally, there is one which has a structure in which left and right down tube main bodies are welded and joined by plural cross pipes extending in a vehicle width direction.
However, in the case in which the aluminum pipe material formed by extrusion molding is adopted, a thickness of the pipe material is constant in a direction of extrusion. Therefore, there is a concern that, if a thickness of a portion requiring large strength is secured, a film thickness of a portion not requiring large strength is also increased and, as a result, an effect of reducing a vehicle weight cannot be obtained despite the fact that the body frame is made of aluminum alloy.
In addition, in the case in which the extrusion molded product is adopted, there is a problem in that bending and deformed molding are difficult, and a degree of freedom with respect to a shape, a size, and the like is low. Therefore, the body frame is formed by welding a large number of linear short molded products, and there is a problem in that strength is fluctuated so much easier for an increase in welded portions, and cost increases.
In addition, in the conventional body frame, since the structure is adopted in which the main pipe and the down tube are welded and joined to the head pipe, respectively, and the left and right gussets are welded and joined thereto, there is a problem in that a structure around the head pipe is complicated and there are many welded portions, strength is easily fluctuated, and cost increases.
Further, as in the conventional down tube, in the case in which the structure is adopted in which the left and right down tube main bodies are welded and joined by the plural cross pipes, strength is fluctuated so much easier for an increase in the welded portions, and cost increases so much more for an increase in the number of components.
Moreover, in the conventional down tube, there is a problem in that deformation due to welding easily occurs, straightening may be performed by post processing in order to increase an assembly accuracy of the engine unit, and productivity is low.
It is an advantage of the present invention to provide a body frame for a motorcycle which, in the case in which a body frame of aluminum alloy is adopted, can reduce a body weight while securing rigidity and strength and can further increase a degree of freedom of a shape, a size, and the like of the frame and prevent fluctuation of strength, and can also reduce cost.
It is another advantage of the present invention to provide a body frame for a motorcycle which can prevent fluctuation of strength by reducing welded portion around a head pipe and can also reduce cost.
It is yet another advantage of the present invention to provide a body frame for a motorcycle which can prevent fluctuation of strength by reducing welded portions of a down tube, and can also reduce cost and, moreover, can improve productivity by making straightening work by post treatment unnecessary.